Improve parsing of counter/counters CSS content functions (#705)

Instead of exploding the string based on the argument delimeter (,)
parse the string using a regex that pulls the arguments into a
"matches" array ($args). This allows for the presence of a comma
in the joining string argument of the counters() function.

The output from the counters() function was also updated. First
the joining string is now only used between the counter instances
where previously it was also appended to the end of the output.
Second, elements that reset the counter but do not actually
increment it are skipped. This matches the output generated by
standards-compliant browsers.